Add separate light/dark mode accent colors

Adds an option in Appearance settings to use different accent colors
in light and dark mode, with a toggle to keep a single shared color
(the default, preserving existing behavior).

- New synced settings keys: accentColorDark, customAccentColorDark,
  useSeparateDarkAccentColor; dark values fall back to the light
  selection until explicitly set, so no migration is needed
- resolvedAccentColor returns a dynamic platform color (UIColor trait
  provider / NSColor appearance provider) when the toggle is on, so
  the root tint and all direct readers adapt to light/dark
  automatically without consumer changes
- Accent Color section shows two stacked preset+custom grids with
  Light/Dark headers when enabled, extracted into AccentColorGrid;
  CustomAccentColorButton now takes bindings so the macOS color panel
  edits whichever target was last opened
- New DarkAccentColorTests covering sync contract, default-off state,
  fallbacks, and toggle-off resolution

Claude-Session: https://claude.ai/code/session_0154KH8RAVAvm6iVanhmoW8o

Yattee/Core/SettingsManager.swift

@@ -29,6 +29,9 @@ final class SettingsManager {
     var _theme: AppTheme?
     var _accentColor: AccentColor?
     var _customAccentColor: String?
+    var _accentColorDark: AccentColor?
+    var _customAccentColorDark: String?
+    var _useSeparateDarkAccentColor: Bool?
     var _showWatchedCheckmark: Bool?
 
     // Playback
@@ -419,6 +422,9 @@ final class SettingsManager {
         _theme = nil
         _accentColor = nil
         _customAccentColor = nil
+        _accentColorDark = nil
+        _customAccentColorDark = nil
+        _useSeparateDarkAccentColor = nil
         _showWatchedCheckmark = nil
         _preferredQuality = nil
         _cellularQuality = nil